The intestinal epithelium is the most rapidly self-renewing tissue in adult mammals. We have recently shown that Lgr5 (Leucine-rich repeat-containing G protein-coupled receptor) is expressed in intestinal stem cells by an in vivo genetic lineage tracing strategy. In the past, extensive efforts have been made to establish primary small intestinal culture systems. However, no defined, reproducible and robust culture system had been developed. To establish such a system, we screened for optimal growth factor combinations based on genetic evidence of self-renewal regulation, differentiation, and carcinogenesis of intestinal stem cells. Here, we describe methods that we have established for the isolation and culture of primary small intestinal epithelial stem cells. In this culture system, isolated crypts form "organoid structures" with a histological hierarchy recapitulating in vivo small intestinal epithelium. Single isolated Lgr5+ intestinal stem cells also form these organoid structures, in which stem cells are maintained by self-renewal and give rise to all lineages of the intestinal epithelium. This culture system is particularly useful for studying the regulation of intestinal stem cell self-renewal and differentiation.